1. Field of the Invention
The present invention relates to a thermal reed switch assembly which is capable of performing a bandless operation with adjustment of an operating temperature and achieving a widened difference range between its operating temperature and reset temperature by selectively establishing such two temperatures at desired points respectively.
2. Description of the Prior Art
In the conventional thermal reed switch assembly of bandless operation type combined with a thermal magnetic member and a permanent magnet so as to be turned off (or on) above a certain temperature or turned on (or off) below such temperature, it is necessary to select each time a proper thermal magnetic member of the material composition that indicates required characteristic change relative to the operating temperature, and accordingly a plurality of thermal magnetic members of different material composition need to be prepared for individual operating temperatures. Therefore a problem has been existent heretofore that unless various kinds of thermal magnetic members are prepared, it is impossible to produce a satisfactory thermal reed switch assembly which functions at any desired operating temperature.
Furthermore, since merely one thermal magnetic member is employed, once the operating temperature is established, the reset temperature is determined at a given lower point (e.g. lower by 3.degree.-4.degree. C.), whereby it is rendered impossible to select desired temperatures respectively or to increase the temperature difference therebetween to a desired range.
An exemplary thermal reed switch assembly known heretofore is so composed as shown in FIGS. 9 (a) and 9(b).
FIG. 9 (a) is a sectional view of a conventional break type reed switch assembly (whose contacts are disconnected above its operating temperature), and FIG. 9 (b) graphically shows the principle of operation of the thermal reed switch with the relationship between the temperature and the magnetic flux which flows in the contact region of the switch (where the flux flowing rightward in the drawing is positive). In FIG. 9(a), the reed switch 6 has a contact region 5 in the axial center of an elongated glass receptacle, and two ferromagnetic reed pieces 3 and 4 supported in the receptacle extend axially from the contact region 5 in mutually opposite directions. Designated at 1a and 1b are permanent magnets whose magnetic transition point (Curie point) is sufficiently higher than the operating temperature, and a thermal magnetic member 2 having a Curie point equivalent to the operating temperature is disposed opposite to the contact region 5. On the two sides of the thermal magnetic member 2, the permanent magnets 1a and 1b are attached firmly to the reed switch 6 while being opposed to the reed pieces 3 and 4 in such a manner that the direction of magnetization thereof becomes coincident with the axial direction of the reed switch. Assume now that a temperature rise occurs in the above arrangement. Since the thermal magnetic member 2 remains ferromagnetic under its Curie point, the magnetic flux flowing from the magnet 1b to the magnet 1a mostly passes through the magnetic member and thereby forms a magnetic circuit which extends by way of the reed pieces 3 and 4, so that the magnetic flux flowing in the contact region 5 provides a great force of attraction sufficient to retain the contacts at the attracted positions thereof against the elasticity of the reed pieces 3 and 4. Such magnetic flux flowing in the contact region 5 gradually decreases in accordance with temperature rise, as represented by the curve shown in FIG. 9 (b). That is, the saturation flux density of the thermal magnetic member 2 is reduced and consequently the magnetic flux flowing therein leaks out to the contact region 5. The leakage flux is directionally reverse to the magnetic flux passing through the contact region 5 and, as a result of the mutual cancellation thereof, the flux flowing in the contact region 5 decreases. When the flux thus reduced reaches the value D.O, the elasticity of the reed pieces 3 and 4 overcomes the remaining force of attraction and thereby separates the contacts from each other. The temperature (represented by OT) at this moment is the operating temperature. On the other hand, when there occurs a temperature fall from a point higher than the operating temperature, the magnetic flux fails to provide, even after arrival of the temperature at OT, a great force of attraction sufficient to overcome the elasticity of the reed pieces 3 and 4. And upon arrival at the temperature represented by RT (reset temperature), the magnetic flux P.I capable of providing a great force of attraction sufficient to attract the contacts against the elasticity of the reed pieces 3 and 4 comes to flow again in the contact region 5.
In the assembly of the above-mentioned structure where merely one thermal magnetic member is employed, it is difficult to achieve an increased difference (temperature hysteresis) between the operating temperature and the reset temperature by selectively establishing them at desired points respectively, and fine adjustment is not executable either.
There is known one prior art as disclosed in Japanese Patent Laid-open No. 35425/1980, wherein two ring-shaped thermal magnetic members having mutually difference Curie points are arrayed axially and interposed between permanent magnets. In this example, however, there exists a disadvantage that when one magnetic member of a lower Curie point loses its magnetism, no magnetic flux is generated to consequently bring about failure in retaining the switch in its on-state.